Two ideas for you:
- Slightly different, but I experienced this with my wifi-connected Samsung printer: its sleep-mode affects also wifi, so after a while the printer loses connectivity via wifi. That can be changed in printer's admin menu, so that the wifi network capability stays active also in the sleep mode. Sounds a bit different than your problem (as you can still ping the printer from the router itself), but your problem might be something similar.
- I have seen mentioned that some printers do not like old WPA encryption and prefer WPA2, so that using WPA would lead into wifi connectivity loss sooner or later.
Make also sure that you have the newest firmware in the printer itself.
You being able to ping from router itself but not from a client PC might also hint toward
- DNS/DHCP problem (if you are pinging with name instead of IP)
- your firewall blocking the connection somehow, but that should not happen inside LAN
- some kind of wifi isolation setting that prevents wifi devices from seeing each other.